I suspected her simply because she’s the sister that is the most frequent visitor to our house. Fiancée has 5 sisters, but this is the one who she sees most.
To answer the question about whether it could be a collaborative effort, I’ve known fiancée for longer than we’ve been together. Going back through our history as coworkers (we met when we were partnered together on an ambulance, a few years before we started dating), friends, and everything before we got together, as since, I’m inclined to say “no.”
She’s one of those anal retentive people who will walk $1 back into a store if they give her the wrong change. She once called a McDonald’s after we left the drive thru to tell them that they gave us one too many McNuggets. If I hadn’t witnessed that, I’d have never believed it. Her core is honesty and integrity.
I suppose it’s technically conceivable, but nothing I know about her would lead me to think so.